The present invention relates to lasers and nonlinear frequency conversion techniques and, particularly, to a technique for producing modulated laser emission by using noncritically phased matched sum frequency generation.
Solid state lasers are a class of lasers which contain a solid state gain element. The gain element generally consists of a host material which can be either a crystalline or amorphous or glass-like material and a dopant or impurity ion distributed within the host material. The dopant ion, which is typically a transition element or a rare earth element, is the primary determinant of the wavelength or wavelengths at which the laser can emit radiation. Typically, solid state lasers operate in the infrared region, that is, between 700 nm and 3 .mu..
Trivalent neodymium (Nd.sup.3+) ions are a commonly used rare earth dopant ion that emits at several wavelengths in the infrared. The exact wavelength of the neodymium laser transitions are dependent on the host material. For the Nd.sup.3+ host crystal YAG, which is yttrium aluminum garnet and is a commonly used trivalent neodymium host, several emission wavelengths are well-known and are approximately 946 nm, 1.064 .mu.m, and 1.318 .mu.m. Typically, the highest gain transition for neodymium ions is the 1.064 .mu.m transition. The 946 nm transition has a terminal level in a thermally populated Stark level of the ground state and is generally an inefficient laser transition. The 1.318 .mu.m transition in many trivalent neodymium host materials is much weaker than the 1.064 .mu.m transition and is therefore both less efficient and is difficult to scale to higher output power. However, in some hosts, most notably YAlO.sub.3 (or YALO), the 1.318 .mu.m stimulated emission cross-section is comparable to that at 1.064 .mu.m, and it is possible to obtain laser emission at 1.318 .mu.m with reasonable efficiency and output power.
The host Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12 or GSGG produces efficient laser output when doped with trivalent neodymium ions. More importantly, when this host material is co-doped with the Cr.sup.3+ ion, the neodymium transition intensities are not at all effected but the slope efficiency for pumping with broadband radiation increases dramatically. This is a consequence of the broadband absorption of the Cr.sup.3+ ions and the efficient energy transfer between the Cr.sup.3+ ions to the trivalent neodymium ions. The energy transfer process results in populating the upper laser level for the Nd.sup.3+ ions. See, for example R. Scheps Applied Physics Letters, vol. 59, p. 1287, 1991.
Co-doped GSGG, or Cr,Nd:GSGG, can be optically pumped over a wide wavelength range. It has an extremely strong peak absorption at approximately 655 nm with an absorption bandwidth of approximately 100 nm. The 1.318 .mu.m transition in co-doped GSGG can be obtained efficiently, therefore, when pumping in the red wavelength region near 655 nm.
A laser consists of a gain element contained between at least two reflective mirrors. Typically these mirrors are dielectric coatings and may be placed on the gain element itself. The region between the reflective mirrors is termed the laser cavity. Because optical intensity within the cavity is reflected back and forth, the mirrors that bound the laser cavity are sometimes termed a "laser resonator", or a "resonator". In order for light within the laser cavity to reflect between the cavity mirrors without walking out of the resonator it must be contained within a stable resonator mode. The resonator mode is a mathematical description of the spatial distribution of the optical intensity within the resonator.
Much information on resonator modes has been written and appears in basic texts on lasers, see for example A. Siegman, Lasers, University Science Books, Mill Valley, 1986. Many different, stable resonator modes can exist in a given resonator. The specific features of the resonator modes are determined by the types and placement of the mirrors that define the laser cavity, and the shape and refractive index of materials contained within the laser cavity, such as the gain element. The mode size typically changes within the laser resonator.
The most desirable stable resonator mode is called the TEM.sub.00 mode. This mode is the lowest order transverse mode in a stable resonator and is described by a Gaussian intensity distribution. It can be focussed to a small spot size and is therefore desirable for applications such as nonlinear optical conversion. Modes can go through a focus inside the resonator. The focus is called a waist and represents the local minimum in the mode radius. Examples of modes are concentric, hemispherical and confocal.
It is often desirable for numerous applications to use a laser that emits in the visible region of the spectrum. Because of the convenience of the solid state laser gain medium compared to either gaseous or liquid gain media, techniques have evolved to convert the infrared fundamental radiation from the solid state medium to visible radiation. Nonlinear optical conversion is commonly used to produce visible radiation from solid state lasers operating in the infrared. Wavelengths in the blue are of particular interest for applications such as display technology, optical data storage, underwater applications and communications.
Nonlinear optical conversion can occur in a variety of media but is most conveniently effected in a solid state nonlinear crystal. The nonlinear optical conversion process, of which several are known, converts radiation from one wavelength to another. Examples of nonlinear processes: are second harmonic generation, optical parametric oscillation and sum frequency generation (SFG). An important parameter for determining the efficiency by which the nonlinear crystal converts radiation at one wavelength to another is called the phase matching condition. Optimum conversion from the fundamental wavelength to the converted wavelength will occur when the wave vector mismatch between the fundamental wave and the generated wave is zero. This condition is termed "phase matching". Phase matching may be achieved in an anisotropic crystal by a suitable choice of direction of propagation and polarization relative to the crystalline axes.
Two different types of phase matching can occur in crystals. To understand the difference between these two types one must first recognize that the nonlinear process occurs through the interaction of two fundamental waves within the nonlinear medium. For optical parametric oscillation and sum frequency generation, these two waves represent the two fundamental wavelengths. For second harmonic generation, there is only one fundamental wavelength but the two waves that are interacting in the nonlinear medium in this case are distinguished by their polarization. Type I phase matching refers to the process where the two fundamental waves have parallel polarization. Type II phase matching occurs when the fundamental waves have orthogonal polarization.
Phase matching is achieved as a result of the dispersion of the nonlinear crystalline host. Dispersion refers to the dependence of the refractive index of a given material on wavelength. Therefore, a phase matched crystal is one which is phase matched for a specific nonlinear operation. For example, for second harmonic generation in a Nd:YAG laser, the fundamental wavelength at 1.064 .mu.m is converted to 532 nm emission. One requires that the refractive index at 1.064 .mu.m and the refractive index in the same material at 532 nm be such that the phase relationship between the fundamental wavelength and the generated second harmonic wavelength remain unchanged as the two waves propagate along the length of the crystal.
When phase matched second harmonic generation is achieved by propagating the fundamental wavelength along a direction different from a principal axis of a birefringent crystal, it is termed "critical phase matching". When critical phase matched second harmonic generation is used with a focused beam, there is a phase mismatch of the wave vector for small deviations from the phase matched direction due to the finite divergence of the beam. Because the efficiency of the nonlinear conversion process is a function of the power density within the nonlinear crystal, focusing is generally desirable in order to achieve high conversion efficiency.
When the phase matching angle is 90.degree. for a particular nonlinear process in a given material, it is termed "noncritical phase matching" (NCPM). In such a case, effects of beam divergence vanish; that is to say, a strongly focused beam in an NCPM crystal does not have the phase mismatch problems as is evident in critical phase matching. In addition, the walk-off angle, which is the direction of energy flow of the fundamental and second harmonic beams, is zero, meaning that these two beams propagate collinearly within the crystal. It is obvious, then, that NCPM is the most favored and desirable means of operating a nonlinear material. One means by which NCPM can be obtained is by adjusting the temperature of the nonlinear crystal to the point where the refractive index of the fundamental wavelength equals that of the second harmonic wavelength.
Second harmonic generation, which has been discussed above, is a special case of a more general nonlinear optical conversion process known as sum frequency generation. In second harmonic generation, two waves of the same wavelength are combined to produce a single wave of a wavelength one half of the original fundamental wavelength. In sum frequency generation, two fundamental waves of different wavelengths are combined to produce a single wave. The wavelength produced by sum frequency generation is determined by the following equation: ##EQU1## where .lambda..sub.1 represents one of the fundamental wavelengths, .lambda..sub.2 represents the second of the fundamental wavelengths and .lambda..sub.3 represents the converted or summed wavelength.
It can be seen, then, that second harmonic generation is a degenerate case of sum frequency generation, where .lambda..sub.1 =.lambda..sub.2. The fundamental principles of nonlinear optics summarized briefly above are well known and are discussed in detail in the literature. See for example G. D. Boyd and D. A. Kleinman, Journal of Applied Physics, vol. 39, p. 3597, 1968.
Although second harmonic generation or "doubling" can be an efficient means for conversion to the blue, the nonlinear optical material KTiOPO.sub.4 (KTP) is non-critically phase matched at room temperature for SFG using fundamental wavelengths at 1.318 .mu.m and 659 nm. See for example D. W. Anthon, G. J. Dixon, M. G. Ressl, and T. J. Pier, SPIE Proceedings, vol. 898, p. 68, 1988. The generated wavelength is 439 nm. It should be noted that KTP is a mature and well characterized material, unlike some of the nonlinear crystals that are required to produce 439 nm through second harmonic generation. In addition, because of the NCPM nature of the sum frequency process, this process will have an exceptionally wide angular and temperature bandwidth for NCPM SFG.
Typically, SFG requires two different laser sources. Because the efficiency of the sum frequency generation process depends on the power density within the optical crystal for cw operation, one requires an extremely small focus spot size within the nonlinear SFG crystal. A certain element of complexity is involved in using two separate lasers and this is a situation that is best avoided.
One technique for avoiding the use of two separate lasers for the sum frequency generation process is to use a laser to pump a gain element which emits one of the two fundamental wavelengths. If the wavelength of the pump laser matches the wavelength required for the second fundamental, one can combine the pump laser output with the pumped laser output to produce sum frequency generation. The only laser that is required to initiate and sustain this process is the lone pump laser. This technique was used for a diode pumped Nd:YAG laser. See for example W. P. Risk, J.-C. Baumert, G. C. Bjorklund, F. M. Schellenberg and W. Lenth, Applied Physics Letters, vol. 52, p. 85, 1988.
In this type of sum frequency generation a laser diode operating at 808 nm is used to pump a Nd:YAG laser which operates at 1.064 .mu.m. The residual or unabsorbed 808 nm pump light is then circulated within the Nd:YAG laser resonator cavity which also includes a sum frequency generating KTP crystal. In such a system there is only one active laser, the laser diode. The Nd:YAG is optically excited by the laser diode and in essence serves as a frequency conversion device to convert some of the 808 nm light to 1.064 .mu.m light. In this manner one might conclude that sum frequency generation is achieved with the use of only one active laser. A patent by Baumert et al., U.S. Pat. No. 4,791,631 describes this concept in detail.
A variation of this type of sum frequency generation process is to use an additional laser diode or laser diodes which do not pump the Nd:YAG directly but are used to introduce additional 808 nm light into the laser resonator. The laser resonator also contains the Nd:YAG crystal and the KTP nonlinear crystal. In this case a separate laser diode is used to pump the Nd:YAG laser. This concept has been described in detail by Dixon et al. in U.S. Pat. No. 4,879,723. However, laser diodes tend to be a poor choice for applications where moderate power (on the order of 1 Watt or greater) is required. This arises from the fact that higher power laser diodes do not operate in a single spatial mode with good spatial coherence. Spatial coherence and single mode operation allow the fundamental beam to be focused to a small waist in the nonlinear crystal for efficient SFG.
In general, solid state lasers, which by common usage are distinguished from laser diodes, are not effectively modulated at high modulation rates by modulating the pump power. The maximum modulation frequency is limited to the inverse of the lifetime of the dopant ion. For trivalent neodymium ions in YAG, the lifetime is 230 microseconds. Thus the maximum modulation rate is limited to approximately 4 kHz. Since modern communications systems require hundreds of megahertz to tens of gigahertz modulation rates, a solid state laser would have to be modulated externally to produce such rapid modulation. This can be accomplished by operating the solid state laser cw and having an electrooptic, acoustooptic or mechanical means to interrupt the beam. Duty factors for many communication systems are very low (the duty factor is the laser on time to laser off time). For pulse position modulation communications formats, the duty cycle is less than 1%. Therefore, operating the laser cw and using external modulation can be a highly inefficient means of operation. When primary power is limited, for example in remote installations such as a satellite or an unmanned vehicle, electrical efficiency is an important consideration.
The sum frequency generation process lends itself effectively to modulation of visible radiation at the high data rates characteristic of laser diodes, but using the advantages of solid state laser gain media. An example of this method is the following. If the output of a diode pumped Nd:YAG laser operating at 1.064 .mu.m is mixed with the diode pump light, the intensity of the resulting 459 nm output will be linearly dependent on the intensity of the 808 nm fundamental. By modulating the 808 nm laser diode pump light at high frequencies, for example several hundred megahertz, an approximately cw output at 1.064 .mu.m is produced, but the sum frequency generated output at 459 nm will be modulated at the same frequency as the 808 nm pump light.
The patent referred to previously by Baumert et al. describes a technique for a diode pumped Nd:YAG laser in which modulated output at 459 nm is produced. The laser diodes are operated at 808.5 nm, which is the ideal wavelength for pumping the Nd:YAG gain element. The patent referred to previously by Dixon et al. describes an essentially similar technique for producing high modulated 459 nm radiation from a diode pumped Nd:YAG laser. However, the concepts introduced by Baumert et al. and Dixon et al. suffer from two serious flaws. The first is that the laser diode wavelength is determined by the absorption spectrum of Nd:YAG; that is, the strongest absorption line occurs at 808.5 nm. However, the best wavelength for the SFG process, by which 1.064 .mu.m and laser diode emission are combined to produce 459 nm emission in an NCPM KTP crystal, is not 808.5 nm but 806 nm.
Unfortunately, 806 nm is not an effective wavelength for pumping Nd:YAG. Therefore, in the concepts proposed by Baumert et al. and Dixon et al. diode laser emission must be generated at a wavelength which is not optimum for either or both Nd:YAG pumping and NCPM SFG in KTP, or introduce additional laser diodes. By introducing additional laser diodes one or more diodes can be used for 808.5 nm pumping of the Nd:YAG laser crystal while other diodes can be used to produce 806 nm radiation for 459 nm SFG. However, this system becomes cumbersome and requires numerous active lasers to produce the 459 nm SFG.
A more serious problem with both the Baumert et al. and Dixon et al. concepts is that the intracavity intensity at the approximately 808 nm fundamental wavelength is limited by absorption due to the Nd:YAG gain element contained within the cavity. As mentioned previously, intracavity SFG is required to produce efficient 459 nm output under cw operation because of the much higher circulating powers contained within a resonant cavity. By circulating 808 nm power within a cavity that also contains an absorbing material, the ultimate intensity that can be obtained at 808 nm is limited. This in turn limits the SFG conversion efficiency, which depends linearly on the 808 nm power.
A patent by Dixon, U.S. Pat. No. 5,142,542, discusses yet another technique for intracavity sum frequency generation by which a solid state laser gain element contained within a resonator is optically pumped. The optical pump radiation resonates within the laser cavity as well so that the cavity is doubly resonant for both wavelengths. That is, it is resonant for both the pump wavelength and the wavelength emitted by the laser gain element. In spite of the resonance for the two wavelengths, the presence of the solid state laser gain element contained within the doubly resonant cavity path through which the pump radiation must also resonate introduces strong absorption losses and reduces the intracavity power at the pump wavelength.
Note that the sum frequency generated power P.sub.3 is given by the following expression equation (2): EQU P.sub.3 =k.sub.m P.sub.1 P.sub.2 ( 2)
where P.sub.1 and P.sub.2 are the two fundamental powers passing through the nonlinear sum frequency generating crystal, and k.sub.m is a constant determined by, among other things, the physical properties of the nonlinear sum frequency generating crystal and the beam focusing and phase matching conditions. It can be readily verified that for a fixed total power, that is, where P.sub.1 +P.sub.2 is a constant, P.sub.3 is maximized when P.sub.1 =P.sub.2.
Absorption of the intracavity power from the pump radiation (P.sub.1 for example) by the laser gain element increases the intracavity power at P.sub.2. But P.sub.2 is increased while P.sub.1 is decreased, so that the resulting P.sub.3 is lower than it would be if there were no absorption. Although the patent by Dixon cited above discloses a doubly resonant cavity, the presence of the laser gain element introduces absorption loss in the feedback path for P.sub.1, the laser pump radiation, and reduces the magnitude of P.sub.3.
Other versions of intracavity sum frequency generation at 1.06 .mu.m and 808 nm have appeared in the literature. See for example W. P. Risk and W. Lenth, Applied Physics Letters, vol. 54, p. 789, 1989; and P. N. Kean and G. J. Dixon, Optics Letters, vol. 17, p. 127, 1992. However, neither of these published approaches provide a separate resonant path for the pump radiation relative to the intracavity laser gain element which absorbs this pump radiation. In the reference by Kean and Dixon cited above, an optically thin Nd:YAG slab is used to mitigate but not eliminate the absorption problem.
Sum frequency generation in an external cavity, see for example, W. P. Risk and W. J. Kozlovsky, Optics Letters, vol. 17, p. 707, 1992, circumvents the absorption problem but introduces alignment and mode matching difficulties that are not inherent in an intracavity sum frequency generation concept in which the laser gain element is also contained within that cavity. Additional variations of the sum frequency generation process using 808 nm and 1.06 .mu.m to produce 459 nm radiation are reported by J. C. Baumert, F. M. Schellenberg, W. Lenth, W. P. Risk and G. C. Bjorklund, Applied Physics Letters, vol. 51, p. 2192, 1987 and W. Risk, J.-C. Baumert, G. C. Bjorklund, F. M. Schellenberg and W. Lenth, Applied Physics Letters, vol. 52, p. 85, 1988. However, these embodiments suffer from the same problem of reduced intracavity intensity at the pump wavelength (which is 808 nm in this case) due to absorption by the intracavity Nd:YAG gain element. The Nd:YAG gain element is required by the above referenced configurations to remain in the cavity in order to produce 1.06 .mu.m radiation.
To produce efficient SFG for the process where 806 nm and 1.064 .mu.m radiation are summed within a laser cavity to produce the 459 nm output, a technique using a two-branched coupled cavity resonator was designed. This concept is the subject of the above referenced patent applications by R. Scheps U.S. Patent and Trademark Office Ser. Nos. 08/108,131 (U.S. Pat. No. 5,333,142) and 08/183,212. In this two-branched coupled cavity device the Nd:YAG is kept separate from the feedback path which resonates the 806 nm intracavity optical flux. Therefore, one can have very high intracavity flux at both 1.064 .mu.m and 806 nm without Nd:YAG absorption reducing the flux at 806 nm. The inventive concepts described in the above referenced Scheps patent applications Ser. No. 08/108,131 and 08/183,212 are useful for cw or pulsed operation. However, when used with solid state gain media, high modulation rates of the SFG output are not possible for the reasons described above.
Thus, in accordance with this inventive concept a continuing need has been found in the state of the art for a technique for intracavity sum frequency generation to produce visible output at high modulation rates using a nonlinear crystal composed of KTP which is non-critically phase matched, efficient, scalable to high power, insensitive to alignment problems, arises from a single laser source, and contains no elements within the cavity that reduce the intracavity power at either of the two fundamental wavelengths.